1. Field of the Invention
The invention relates generally to the field of filtering contaminants from molten metals and, more particularly, to a technique for separating non-metallic inclusions and contaminants from molten ferrous metal as it is flowed.
2. Description of the Prior Art
A variety of techniques are known for filtering molten metal for various purposes. For example, U.S. Pat. No. 3,981,352, discloses a filter formed by a multiplicity of substantially spherical refractory particles. The particles are held together in a bonded assembly by means of a ceramic binder, the binder substantially coating the particles. Openings are defined between adjoining coated particles to permit a flow of molten metal through the filter. The patent states that the binder is of a type having an affinity for dross and slag constituents in the molten metal whereby the constituents are removed during their passage through the filter.
U.S. Pat. No. 4,007,923 discloses a technique for treating molten aluminum and aluminum alloys to remove solid and gaseous impurities. The metal is flowed through a series of successively arranged purification stages. One of the stages includes a plurality of flat adsorption plates formed of a refractory material that are spaced apart in parallel relation to each other. The molten metal is directed between the plates where impurities in the form of minute particulate matter having an affinity for the material of the plates will adhere to the reactive surfaces of the plates. The '923 patent also discloses a filtration stage employing a porous ceramic filter comprised of small, fused alumina particles that are bonded together with a vitreous material and fired.
U.S. Pat. No. 4,165,235 discloses a technique for inline degassing and filtration of molten metal, especially aluminum, where a plate-like porous ceramic foam filter is disposed in a horizontal orientation. An array of drilled holes extend completely through the filter. Molten metal flows downwardly through the filter, while fluxing gas is passed upwardly through the filter in countercurrent contact with the molten metal. The '235 patent states that although the holes are substantially larger than the individual pore size of the filter, the fluxing gas flows through the holes while substantially all of the molten metal passes through the filter body. In effect, the holes provide a pressure relief for the fluxing gas.
Despite the teachings of the prior art, certain concerns have not been adequately addressed. One of these concerns relates to the type of metal that can be filtered properly. The '352 patent is stated to be of particular importance with respect to the casting of "super alloys" and titanium, while the '923 and '235 patents are stated to be especially adapted for the treatment of aluminum and aluminum alloys. A particular problem exists, however, with respect to ferrous metals such as steel, stainless steel, and ductile iron. The higher temperatures encountered with molten ferrous metals present severe structural problems with many types of filter materials. Further, deoxidation products and exogenous non-metallic inclusions contained within molten steel before casting can build up on the exposed surfaces of filter materials, thereby plugging the filters or at least reducing their flux substantially. Desirably, a plate-like filter would be available that would stand up to the temperatures encountered in the handling of molten ferrous metals while having good filtration efficiency, a high flux, and while avoiding problems associated with the cake-like build-up of non-metallic inclusions.
An additional problem associated with the filtration of molten ferrous metals is that of freeze-up. Because currently available filters greatly restrict the rate of flow of molten ferrous metal in a casting mold, the filter can act as a heat sink, thereby permitting the metal to freeze within the filter. To a foundry, a casting that has not filled because of premature freezing due to the filter represents losses of time in mold preparation and casting and in electrical costs to melt metal for a replacement casting. Two means of reducing the frequency of filter freeze-up are to increase the super heat of the molten ferrous metal and to preheat the filter in the casting mold. Increasing the super heat of the molten ferrous metal is undesirable to foundry operators because of the considerable resulting increase in power consumption. Preheating a filter while it is in the casting mold is impractical.
In the particular case of steel casting in a foundry, one known filter construction designed for casting molten ferrous metals experienced freeze-up of 30% of all such filters during casting. An even worse problem occurs with ceramic foam filters similar to those disclosed in the '235 patent, for almost 100% of solid body foam filters cause a freeze-up problem. In short, ceramic foam filters heretofore have been difficult to use in the casting of molten ferrous metals. Desirably, a filter would be available for the casting of molten ferrous metals that would eliminate, or substantially eliminate, the freeze-up problem.